Hydrogen is currently used in large quantities in the chemical and petroleum industries. In the chemical industry, hydrogen is mainly used to make ammonia for soil fertilizer via the Haber process involving the reaction of nitrogen gas and hydrogen to produce the ammonia. Hydrogen is also used in oil refineries to upgrade petroleum-based compounds in hydroprocessing unit operations that require hydrogen, such as hydrocracking and hydrotreating processes. These processes often aid in lowering the sulfur content of the finished fuel product. The use of hydrogen in refineries is expected to increase due to an increased demand for low sulfur fuels driven by environmental concerns.
The literature describes a variety of different processes to make hydrogen, many of which are the subject of significant research and development efforts. Some of the processes disclosed include biomass pyrolysis or gasification and biological processes, such as bacterial fermentation and enzymatic hydrogen production. Electrolysis is another technology for making hydrogen in which water is decomposed into oxygen and hydrogen. Currently, however, hydrogen is produced commercially from fossil methane. Such production processes are inexpensive and the natural gas feedstock for the process is widely available and relatively low cost.
The production of hydrogen from methane includes a reforming reaction in which the methane is converted to carbon monoxide and hydrogen. A commonly utilized reforming reaction is steam methane reforming (SMR) that converts the methane into hydrogen and carbon monoxide in the presence of steam. Among other known reforming reactions is autothermal reforming, which uses oxygen and carbon dioxide or oxygen and steam in a reaction with methane to form carbon monoxide and hydrogen. Reforming is followed by one or more water gas shift (WGS) reactions in a reactor in which the carbon monoxide and hydrogen are converted to carbon dioxide and additional hydrogen. Hydrogen is then recovered from an outlet stream of the WGS reactor. An example of such a recovery technique is pressure swing adsorption (PSA), which is used to separate gas species from a mixture of gases under pressure using adsorbent materials such as zeolites, molecular sieves or activated carbon. The adsorbent material absorbs the target gas species at high pressure and the separation relies on the different affinity of various gas species in the gas stream. As the pressure is lowered, the target gas desorbs. When used to recover hydrogen, pressure swing adsorption adsorbs hydrogen and the desorption results in a stream concentrated in hydrogen. A stream comprising non-adsorbed species is also generated by pressure swing adsorption, which is often referred to as a purge gas stream.
In existing commercial operations, the purge gas stream is burned to provide process heat. Such heat integration is advantageous as many of the unit operations in hydrogen production operate at high temperature. In a typical hydrogen production process, the purge stream is fed to an SMR furnace that provides heat energy for the methane reforming reaction.
The composition of the purge gas stream varies depending on the operating parameters in the upstream reforming. Components present in this stream may include hydrogen and additionally carbon dioxide, carbon monoxide and/or methane. Hydrogen is present since its recovery in the pressure swing adsorption or other hydrogen recovery operation is incomplete.
Despite the advantages of heat integration, the presence of hydrogen in purge streams is a significant drawback. Hydrogen is costly and simply burning it for heat production is an inefficient use of this component. This in turn reduces the economics of the process. Accordingly, there is a need in the art to better utilize constituents of the purge gas, particularly hydrogen, without significantly disrupting the operation of the hydrogen production process.